The present invention relates to mass flowmeters and related methods and apparatus, and more particularly, to methods, apparatus and computer program products for determining status of a mass flowmeter system.
Many sensor applications involve the detection of mechanical vibration or other motion. Examples of sensors that utilize such motion detection include Coriolis mass flowmeters and vibrating tube densitometers. These devices typically include a conduit or other vessel that is periodically driven, i.e., vibrated. Properties such as mass flow, density and the like associated with a material contained in the conduit or vessel may be determined by processing signals from motion transducers positioned on the containment structure, as the vibrational modes of the vibrating material-filled system generally are affected by the combined mass, stiffness and damping characteristics of the containing conduit or vessel structure and the material contained therein.
A typical Coriolis mass flowmeter includes one or more conduits that are connected inline in a pipeline or other transport system and convey material, e.g., fluids, slurries and the like. in the system. Each conduit may be viewed as having a set of natural vibrational modes including, for example, simple bending, torsional, radial and coupled modes. In a typical Coriolis mass flow measurement application, a conduit is excited at resonance in one of its natural vibrational modes as a material flows through the conduit, and motion of the conduit is measured at points along the conduit. Excitation is typically provided by an actuator, e.g., an electromechanical device, such as a voice coil-type driver, that perturbs the conduit in a periodic fashion. Exemplary Coriolis mass flowmeters are described in U.S. Pat. No. 4,109,524 to Smith, U.S. Pat. No. 4,491,025 to Smith et al., and Re. 31,450 to Smith.
Unfortunately, the accuracy of conventional Coriolis mass flowmeters may be compromised by nonlinearities and asymmetries in the conduit structure, motion arising from extraneous forces, such as forces generated by pumps and compressors that are attached to the flowmeter, and motion arising from pressure forces exerted by the material flowing through the flowmeter conduit. The effects of these forces are commonly reduced for by using flowmeter designs that are balanced to reduce effects attributable to external vibration, and by using frequency domain filters, e.g., bandpass filters designed to filter out components of the motion signals away from the excitation frequency. However, mechanical filtering approaches are often limited by mechanical considerations, e.g., material limitations, mounting constraints, weight limitations, size limitations and the like, and frequency domain filtering may be ineffective at removing unwanted vibrational contributions near the excitation frequency.
According to embodiments of the invention, a method of operating a mass flowmeter system including a material-containing conduit is provided. A plurality of motion signals representing motion of the conduit are mode selective filtered to generate a plurality of mode selective filtered motion signals such that the plurality of mode selective filtered motion signals preferentially represent motion associated with a vibrational mode of the conduit. A plurality of time difference estimates is generated from the plurality of mode selective filtered motion signals. A correlation measure is generated from the plurality of time difference estimates. A status of the mass flowmeter system is determined from the generated correlation measure.
The step of generating a correlation measure may include estimating an intercept parameter of a scaling function that relates the plurality of time difference estimates to a plurality of reference time differences representing motion of the conduit at a known mass flow. For example, an augmented matrix including the plurality of reference time differences may be generated, and the plurality of time difference estimates may be multiplied by a pseudoinverse of the augmented matrix to estimate the intercept parameter. Alternatively, the scaling function may be iteratively estimated, e.g., using a least mean square (LMS) estimation procedure.
According to other embodiments of the invention, a correlation coefficient may be estimated from the plurality of time difference estimates, and the mass flowmeter system status may be determined from the estimated correlation coefficient. According to still other embodiments of the invention, an error of estimate from the plurality of time difference estimates, and the mass flowmeter system status may be determined from the estimated error of estimate.
In still other embodiments of the invention, the mass flowmeter system status may be determined by determining a change in the correlation measure. Corrective action may then be taken if the determined change satisfies a predetermined criterion. For example, a mode selective filter used to generate the mode selective filtered motion signals may be modified based on the determined status of the mass flowmeter system. In other embodiments, a pseudoinverse matrix may be generated from a plurality of reference time differences representing motion of the conduit under a known perturbation. The plurality of time difference estimates may be multiplied by the psuedoinverse matrix to estimate a scaling function that relates the plurality of time difference estimates to the plurality of reference time differences. The pseudoinverse matrix may be modified based on the determined status of the mass flowmeter system.
According to still other embodiments of the invention, the mass flowmeter system includes a plurality of motion transducers operatively associated with the conduit. Determining a mass flowmeter system status may include determining a status of a motion transducer from the generated correlation measure.
In other embodiments of the invention, an apparatus comprises a conduit configured to contain a material. A plurality of motion transducers is operatively associated with the conduit and produces a plurality of motion signals that represent motion of the conduit. A signal processing circuit receives the plurality of motion signals, mode selective filters a plurality of motion signals to generate a plurality of mode selective filtered motion signals such that the plurality of mode selective filtered motion signals preferentially represent motion associated with a vibrational mode of the conduit, generates a plurality of time difference estimates from the plurality of mode selective filtered motion signals, generates a correlation measure from the plurality of time difference estimates, and determines a status of the apparatus from the generated correlation measure.
According to still other embodiments of the invention, a computer program product for determining status of a mass flowmeter system including a material-containing conduit is provided. The computer program product includes a computer readable storage medium embodying computer readable program code, the computer-readable program code comprising computer-readable program code that mode selective filters a plurality of motion signals representing motion of the conduit to generate a plurality of mode selective filtered motion signals such that the plurality of mode selective filtered motion signals preferentially represent motion associated with a vibrational mode of the conduit, that generates a plurality of time difference estimates from the plurality of mode selective filtered motion signals, that generates a correlation measure from the plurality of time difference estimates, and that determines a status of the mass flowmeter system from the generated correlation measure.